1. Field of the Invention
The present invention relates to an internal combustion engine and more particularly to a turbo-supercharger for internal combustion engines.
2. Description of Prior Art
A turbo-supercharger for internal combustion engines includes a turbine located in the engine exhaust system for driving a supercharging compressor which is located in the engine intake system. The turbine is driven by the stream of exhaust gas so as to recover the waste energy in the exhaust gas. Where the turbo-supercharger is employed in an automobile engine, problems arise because the automobile engine is operated throughout a wide output range, that is, from idling operation to high power operation. Under medium load operation, as well as at high speed, heavy load operation, there is a substantial exhaust gas flow, so that adequate driving energy is available to the turbine. However, under light load engine operation, the exhaust gas flow is not sufficient to drive the turbine so that the turbine simply produces a drag to the exhaust gas flow without any contribution to the engine output.
In order to eliminate the above problems, it has been proposed to divide the scroll of the turbine casing into two parallel passages and provide a shut-off valve in one of the passages so that only the other passage is used to allow the exhaust gas to pass therethrough under light load engine operation. For example, in Japanese patent application No. 56-116426 filed on July 17, 1981 and disclosed for public inspection on Feb. 3, 1983 under the disclosure No. 58-18522, there is disclosed a turbo-supercharger including a turbine scroll provided with a substantially radially inwardly extending partition wall dividing the interior of the scroll into a main passage of a relatively small crosssectional area and an auxiliary passage of a relatively large cross-sectional area. The auxiliary passage communicates at the upstream end with the main passage through an opening formed in a wall of the main passage. In the opening of the wall, there is provided a shut-off valve which is closed under low speed, light load engine operation. Thus, under low speed, light load operation the exhaust gas is passed only through the main passage. Since the main passage is of a relatively small cross-sectional area, the speed of the exhaust gas flow is increased to thereby increase the turbine speed even under a small exhaust gas flow. In medium and heavy load operations, the shut-off valve is opened and the exhaust gas is allowed to pass through the main and auxiliary passages. It is therefore possible to prevent the speed of the exhaust gas flow from being increased excessively. In the turbosupercharger proposed by the aforementioned Japanese Patent application, there is further provided a turbine bypass valve for bypassing a part of the exhaust gas around the turbine where an over-charging is likely to occur. The bypass valve is located in the turbine scroll at a wall of the main passage.
The turbo-supercharger proposed by the aforementioned Japanese patent application is considered to solve to some extent the problems inherent to the supercharger for automobile engines, because the turbine speed can be increased under low speed, light load engine operation. It should, however, be noted that since the upstream end of the auxiliary passage is opened to the main passage through the opening in the wall of the main passage, the exhaust gas flow cannot effectively be directed to the auxiliary passage even when the shut-off valve is opened and the overall drag to the exhaust gas flow is increased under heavy load operation. Further, the proposed structure is considered disadvantageous in that the shut-off valve and the bypass valve must be provided in the turbine casing casing so that the turbine becomes structurally complicated.